In the design of pneumatic engines, there has existed an historic problem of releasing "back pressure" caused by a return stroke of the engine piston after a firing or compression stroke has occurred. This problem has caused engine designers to employ complicated exhaust valves or to leave a clearance between the piston diameter and the cylinder diameter, so pressurized air could escape during the return stroke.
The mechanics of the pneumatic engine are very simple. When the piston is moving from the intake valve, it is in the compression stroke. When the piston is furthest away from the intake valve it exhausts any pressure left from the compression cycle. When the moving towards the intake valve, it is in the return cycle. This return cycle is where the piston's movement back to the firing position is critical; no pressure buildup should occur. In all designs only the inertia of the rotating components force the piston down during this cycle.
In the prior art, there exist pneumatic engines without a seal on the piston. These designs do not create back-pressure but are very inefficient during the compression stroke because of air loss between the piston and the cylinder wall. If an O-ring type seal, or any seal that seals in both direction, is used between the piston diameter and the cylinder diameter, the compression stroke becomes very inefficient since any compressed air would then exhaust at the top of the stroke. However, when the piston is returning to its firing position it can create up to 5 five atmospheres of back-pressure before the firing sequence begins again. This effect slows down the rotational speed of the rotating components. Thus more inertia and heavier parts such as flywheels are needed to compensate. These effects in other engines that create back-pressure requires an exhaust valve to vent this pressure. If no exhaust valve is used, such seals significantly lower the performance of the engine, and in some cases cause the engine not to function.
The present invention is therefore directed to an engine seal adapted to seal against the piston wall only during the compression stroke, but not during the return stroke, thereby obviating the need for either an exhaust valve or higher mass engine components.